Two pipes A and B can fill a tank in 28 hours and 21 hours, respectively. If both the pipes are opened simultaneously, then the time taken (in hours) to fill the tank is:

Correct option is B

Given:
Time A = 28 hours, Time B = 21 hours
Formula Used:
Total time = \frac{A \times B}{A + B}
Solution:
​$$Time = \frac{28 \times 21}{28 + 21} = \frac{28 \times 21}{49}\\ \ \\Time = \frac{4 \times 21}{7} = 4 \times 3 = 12 hours$$​​
So the correct answer is (b).